Structural Analysis of the Partially Disordered Protein EspK from Mycobacterium Tuberculosis

For centuries, tuberculosis has been a worldwide burden for human health, and gaps in our understanding of its pathogenesis have hampered the development of new treatments. ESX‐1 is a complex machinery responsible for the secretion of virulence factors that manipulate the host response. Despite the importance of these secreted proteins for pathogenicity, only a few of them have been structurally and functionally characterised. Here, we describe a structural study of the ESX‐secretion associated protein K (EspK), a 74 kDa protein known to be essential for the secretion of other substrates and the cytolytic effects of ESX‐1. Small‐Angle X‐ray Scattering (SAXS) data show that EspK is a long molecule with a maximal dimension of 228 Å. It consists of two independent folded regions at each end of the protein connected by a flexible unstructured region driving the protein to coexist as an ensemble of conformations. Limited proteolysis identified a 26 kDa globular domain at the C‐terminus of the protein consisting of a mixture of α‐helices and β‐strands, as shown by circular dichroism (CD) and SAXS. In contrast, the N‐terminal portion is mainly helical with an elongated shape. Sequence conservation suggests that this architecture is preserved amongst the different mycobacteria species, proposing specific roles for the N‐ and C‐terminal domains assisted by the middle flexible linker.